This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A215797 #38 Feb 16 2025 08:33:18
%S A215797 0,1,2,5,90
%N A215797 Numbers k such that k*(k+1)/2 + 1 is a power of 2.
%C A215797 No other terms < 10^6. - _T. D. Noe_, Aug 25 2012
%C A215797 This sequence maps to the Ramanujan-Nagell squares (8*(k*(k+1)/2)+1) and is therefore complete. - _Raphie Frank_, Aug 26 2012
%C A215797 All terms in this sequence follow form floor[2^((2*x - 1)/2)]; x = {0, 1, 2, 3, 7}. - _Raphie Frank_, Mar 03 2013
%H A215797 Eric W. Weisstein, <a href="https://mathworld.wolfram.com/RamanujansSquareEquation.html">MathWorld: Ramanujan's Square Equation</a>
%F A215797 a(n) = -1 + ceiling[sqrt(2^(A060728(n) - 2) - 1)]. - _Raphie Frank_, Mar 31 2013
%F A215797 a(n) = (|(((1+i*sqrt(7))/2)^(A060728(n) - 2) + ((1-i*sqrt(7))/2)^(A060728(n) - 2))| - 1)/2. - _Raphie Frank_, Dec 25 2013
%t A215797 Select[Range[0,1000], IntegerQ[Log[2, 1 + #(#+1)/2]]&] (* _T. D. Noe_, Aug 25 2012 *)
%o A215797 (PARI) for(n=0,100,if(ispolygonal(2^n-1,3),print1(sqrtint(2*2^n-2)", "))) \\ _Charles R Greathouse IV_, Mar 04 2013
%Y A215797 Cf. A060728, A038198 (two references to the Ramanujan-Nagell problem).
%K A215797 nonn,fini,full
%O A215797 1,3
%A A215797 _V. Raman_, Aug 23 2012